1. Field of the Invention
The present invention pertains to a method and apparatus for manufacturing an oxygen-free copper which is suitably used as a material for the electrodes of electron tubes.
2. Prior Art
When copper is melted under ambient conditions, several tens to several thousands parts per million by weight of oxygen is dissolved, and this oxygen remains when the copper returns to the solid state. Such dissolved oxygen causes a so-called hydrogen embrittlement, which results in a lowering of hot-workability and blistering during annealing. This is because hydrogen in the atmosphere diffuses into and through the copper to react with the oxygen dissolved in the copper to produce water vapor, which results in internal defects or surface discontinuities causing blistering.
Furthermore, when some alloying elements are added to copper containing no less than 1O ppm by weight of oxygen, the oxygen reacts with the alloying elements to produce nonmetal inclusions of oxides, which give rise to microscopic defects, detracting from the characteristics of the alloy itself. In addition, since gaseous discharge from the metal is increased, the copper becomes unsuitable as a material for vacuum vessels or electron tubes. Thus, the oxygen in metals has deleterious effects, and hence it is necessary to keep the oxygen content as low as possible when melting copper.
Hitherto known methods for the deoxidization of molten copper are as follows:
(1) the method of adding a small amount of a deoxidizing element such as phosphorus (P) to the molten copper;
(2) the method of coating the molten copper with a carbonaceous coating material such as charcoal;
(3) the method of melting the copper under an atmosphere of a reducing sealing gas such as that of carbon monoxide (CO) gas; and
(4) the method of vaporizing out the dissolved oxygen by melting the copper in a vacuum.
The above methods, however, have the following disadvantages.
In method (1), since the element added as the deoxidizer and the resulting oxide remain in the molten copper and have an adverse effect on the characteristics of the copper, time and labor is required to remove them. In addition, it is very difficult to reduce the oxygen content to less than 1O ppm by weight by this method.
In method (2) in which the oxygen is caused to react with the coating, if the coating is damaged, oxygen is immediately absorbed into the copper, with detrimental effects. Also with this method it is difficult to reduce the oxygen content to less than 5 ppm by weight.
Method (3) has been extensively used on an industrial scale for producing oxygen-free copper containing no greater than 1O ppm by weight of oxygen. This method exhibits a superior deoxidizing performance and makes use of the following deoxidization reaction: EQU CO+[O] in molten Cu.fwdarw.CO.sub.2
However, since the reaction rate is determined by the diffusion of oxygen in the copper, it is slow and hence a long processing time is required. Therefore, a holding furnace of a large volume must be located between the melting furnace and the casting machine, increasing installation and operation costs unduly. Furthermore, inasmuch as the reaction rate is slow, the oxygen removed to the ambient atmosphere and to the containing vessel is absorbed back into the copper to some extent when the oxygen content is reduced, making it difficult to reduce the oxygen content below 3 ppm by weight.
Moreover, in method (4) involving melting under a vacuum, deoxidization is effected according to the following reaction: EQU 2 [O].fwdarw.O.sub.2
In order to reduce the oxygen content below 3 ppm by weight according to this reaction, a high degree of vacuum in the vessel is necessary, and hence a substantial apparatus is required. In addition, since the reaction rate itself is slow, a long processing time is required, thereby significantly increasing manufacturing cost. Furthermore, this process is basically of a batch type and is not applicable to a continuous casting process utilizing a continuous casting machine.